Mercury cell



Sept 7, 1954 J. oosTERMAN 2,688,594

MERCURY CELL Filed Nov. 23, 1949 /va/vcorral/NW6, IN VENTOR 4cm enum/vz .rm/PJ WA4/:mem cfg@ @ffl/amg ATTORNEY Patented Sept. 7, 1954 MERGURY `CELL l.Fan Oosterman, Arnhem, Netherlands, assigner to American linka Corporation, Erika, N. C., a

corporation of Delaware Application November 23, 11949, Serial No. 129,073

Claims priority, application Netherlands December 27, 1948 l Claim. l

This invention relates to an electrolytic cell that is particularly useful in recovering metals and acids from aqueous solution of their salts. More particularly, the invention is directed to an electrolytic cell for electrolyzing aqueous solutions of alkali metal salts according to the amalgam method in order to recover the alkali metal hydroxide and acid from such aqueous solutions.

When electrolyzing aqueous solutions of alkali metal salts whereby an acid is formed at the anode, a diaphragm is commonly employed in order to counteract the diffusion of the acid to the cathode. In order to prevent the accumulation of hydrogen that is being developed at the cathode, it has been proposed to dispose the anode and the diaphragm at a certain slight angle with respect to the horizontally disposed mercury cathode (see British Patent No. 471,912). The disadvantage of such an arrangement is that the distance between the anode and the cathode is not uniform 'throughout the cell due to the angular displacement between the anode and cathode, and consequently the current resistance of the electrolyte is higher at the end of the cell where the anode and cathode are the greatest distance from each other, so that in that portion of the cell the current is considerably smaller than at the other end of the cell where the anode and cathode are separated by the least distance. For that reason it is impossible to operate such an electrolytic cell at the maximum current density which it would be possible to employ at the end of the cell where the anode and cathode are separated by the minimum distance, and consequently it is impossible to operate the cell under conditions of maximum einciency.

According to Aten (U. S. Patent No. 2,230,023) it has been proposed to replace the horizontally disposed mercury cathode by a step-like cathode giving risc to a cascade arrangement in order to avoid too large local differences in current densities. Each step consists of a trough and the supply and discharge-slots of the troughs are arranged in such a way that the cascading iiow or" the mercury takes place in zig-zag fashion. However, this structure is rather complicated and also at times gives rise to difficulties by permitting dead pockets or elds in the flow of mercury, and where the mercury does not ilow suiciently rapidly it taires too much sodium. Due to these local irregularities in flow, some decomposition of the sodium amalgam `may readily occur with the development of hydrogen. Moreover, in the structure illustrated in this patent the ma- (Cl. 20d-219) M terial over which the mercury flows is made of a non-conductive material, or is covered with such material, which thus renders it somewhat more troublesome in obtaining a good current supply to the mercury cathode.

According to the present invention it has Abeen found surprisingly enough that, contrary to previous expectations, it is possible to carry out in a very eincient manner the electrolysis of aqueous solutions of alkali metal salts in an electrolytic cell having an inclined anode and an inclined diaphragm, by causing mercury to flow over a flat or almost flat metal surface or bottom extending parallel to the anode and diaphragm, the slope of the anode and of the bottom of Sthe cell with respect to the horizontal being between 1:50 and 1:5, and preferably about 1:10.

The arrangement of the electrolytic cell according Ito the present invention is such that the metal bottom of the cell is flat or almost flat, and serves as an electrically conductive surface over which the mercury ows, this metal bottom being parallel to the anode, and inclined in the manner indicated in the preceding paragraph.

As a further feature of the present invention, it is also desirable in order to assist in assuring good distribution of 'the mercury over the bottom of the cell (particularly in cases where the electrolytic cell is very long) to provide in said bottom a number of cross grooves at right angles to the direction of flow of the mercury. vThese grooves can be formed in a simple manner by rough planing or machining the bottom of the cell in a direction perpendicular to the direction of the now of mercury, thereby insuring an even flow and distribution of the mercury of the cathode during operation of the cell. These grooves may `be provided over the entire surface of the cell bottom or only over selected portions thereof, as desired.

As a further feature of the invention, the

electrolytic cell according to the present invention may include narrow strips of non-conductive, acid-resisting material ldisposed on the bottom of the cell, and extending in longitudinal direction thereover, i. e., in the direction of the now of mercury between the point of supply and the point oi discharge of the mercury, these strips serving to support the diaphragm (and anode) and hence to maintain the desired parallelism between the diaphragm (and anode) and the flowing mercury cathode.

Numerous other details oi construction of the electrolytic cell of the present invention which cooperate in making it more satisfactory and which are a part of this invention will be apparent from the following detailed description of the preferred form thereof as shown in the accompanying drawing.

In the drawing Figure 1 illustrates schematically a longitudinal section through the electrolytic cell, and Figure 2 represents a crosssectional View taken along the line 2--2 of Fiure 1.

Figure 3 is an enlarged section of the metal bottom showing the cross grooves.

In these gures, I indicates the slightly inclined rectangular bottom of the cell which is made of iron or other electrically conductive material capable of directly supporting in current-conducting relation the flowing mercury cathode. The slope of the bottom may vary from 1:50 to 1:5, but in the preferred arrangement it is about 1:10. The mercury enters the cathode department of the cell via inlet 2 and, after flowing along the inclined bottom I thereby forming a thin uninterrupted layer 4 serving as the flowing mercury cathode, leaves the cathode compartment via outlet 3. Parallel to the bottom I are disposed a diaphragm 5 and a perforated lead anode 6. Current is supplied to the anode and to the flowing mercury cathode through the iron bottom I in convetnional manner by means not shown.

Disposed longitudinally on the bottom I of the electrolytic cell in the direction of iiow of the mercury cathode are a plurality of strips 'I'I' of a non-conductive, acid-resistant material, these strips by virtue of their longitudinal disposition in the cell forming no hindrance to the even flow of the mercury through the cathode compartment. These strips 1, 'I' serve to support the diaphragm 5 (and anode B) in substantially parallel relation with respect to the iron bottom of the cell and hence with respect to the flowing mercury cathode 4.

'I'he upper portion of the electrolytic cell comprises a collecting tank 8 for anolyte, this collecting tank being attached to the bottom I by suitable means such as screws 9, 9. In this way it is possible to replace the diaphragm 5 and the anode 6 when necessary in simple and convenient fashion.

The walls of the upper portion of the electrolytic cell may be manufactured from a non-conductive, acid-resisting material I I, or they can be made of a suitable metal such as iron which is covered with such a material. Such portions of the bottom of the cell as are not covered by mercury may also be provided with a non-conductive, acid-resisting covering material I I. The material II, as well as the strips 1, 'I' may be made of ebonite, glass or other acid-resisting, non-conductive material as desired.

The diaphragm 5 may be made of porous ebonite, or of acid-resistant asbestos fiber, or a mat of glass fiber held together by an acid-resistant binder, or a ceramic material such as corundum or porous tile, or otherwise as desired. For example, excellent results have been achieved with this electrolytic cell when the diaphragm consists of a fabric impregnated with rubber latex, as disclosed in application Ser. No. 128,911, filed November 22, 1949, by Jan Oosterman, now abandoned.

The liquid to be subjetced to electrolysis, e. g., an aqueous solution of an alkali metal salt such as sodium sulfate, may be introduced in known manner under the diaphragm 5 at the lower part l2 of the cathode compartment of the cell, and

caused to flow upwardly against the downflowing mercury. The electrolyte that passes through the diaphragm to the anode collects in the upper portion 8 of the electrolytic cell as acid-enriched anolyte, and overflows from outlet I0. As is well known, it is desirable to supply more solution that is to be electrolyzed than the quantity which passes through the diaphragm. This excess flows through the cathode space and is discharged from outlet I3. If desired, however, the flow of electrolyte through the cathode compartment can be reversed by introducing the liquid to be electrolyzed at I3, with or without discharge of excess at I2.

The applied voltages and current densities per se form no part of the present invention. They may be varied over relatively wide ranges, for example as described in the aforesaid Aten patent, or otherwise.

In order to promote and improve the even ow and distribution of the mercury flowing through the cathode compartment, particularly when the electrolytic cell is rather long, cross grooves Ia are provided either over the etnire bottom of the cell or throughout selected portions thereof. rIhese grooves Ia are at right angles to the flow of mercury and, as indicated above, are provided by rough planing or machining the upper surface of the iron bottom of the cell.

In operation, an aqueous solution of an alkali metal salt, e. g., sodium sulfate or a similar salt of a reactive metal, enters the cell through the opening I2, passes upwardly along the diaphragm 5 and through the diaphragm into the upper portion of the cell where it contacts the anode 6. As it does so, it is electrolyzed and reduced at the mercury cathode to form an amalgam. At the anode, sulfuric acid is formed with the evolution of oxygen which escapes through the open top of the electrolytic cell. The acid liquor is discharged through the outlet I9 and the sodium amalgam is drawn off through outlet 3 from whence it is subjected to a process for recovering the alkali metal from the amalgam in known manner. The mercury thus recovered after separation of the sodium or other alkali metal is recirculated through the cathode compartment.

The electrolytic cell according to the present invention has the following important advantages, in addition to the substantially uniform anode-cathode spacing throughout the cell as well as the relatively simple structure apparent from the drawings. Because of the provision of the slightly sloping bottom of the cell the fiowing mercury cathode is in the form of a very thin layer, so that the mercury content of the cell is particularly low in comparison with other known types of flowing mercury cathode cells. This represents a substantial saving in investment represented by the relatively expensive mercury, while nevertheless insuring a continuous and uninterrupted flow of mercury. Furthermore, the metal bottom of the cell need not be covered and can be manufactured from any suitable, amalgamable metal. Consequently a substantial saving is obtained, and in addition the current supply to the mercury cathode, which over its whole area is in electrically conductive relation to the metal bottom, can be arranged for in a very convenient manner simply by connecting the bus-bars for the cathode to the bottom of the cell.

What is claimed is:

In an electrolytic cell for removing sodium and sulfuric acid from an aqueous solution of soda sulfate, the Combination of an electrically-conductive generally at inclined cell bottom for directly supporting a flowing mercury cathode, an inlet means for said mercury at the upper end of said cell bottom, a discharge means at the lower end thereof, said cell bottom being provided with a plurality of cross grooves positioned at right angles to the direction of ow of the mercury cathode, a perforated lead anode extending parallel to said cell bottom, a permeable diaphragm separating said cell into anode and cathode compartments, said diaphragm being located between said cell bottom and said anode and being also parallel to said cell bottom, said anode having its bottom surface in overall contact with the upper surface of said diaphragm, a plurality of parallel non-conductive acid-resisting spacing strips disposed on the cell bottom extending longitudinally and continuously between the point of inlet and the point of discharge of the flowing mercury to support the diaphragm and the anode in substantially parallel relation with respect to said cell bottom, an inlet at one end of said cathode compartment to supply sodium sulfate solution theerto, an outlet from the anode compartment to remove sulfuric acid therefrom, an outlet for depleted sodium sulfate at the other end of said cathode compartment, said anode, said diaphragm, said spacing strips and said cell bottom all being positioned at an inclination with respect to the horizontal line between 1:50 and 1:5.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Horizontal Mercury Chlorine Cell, FIAT Report No. 816, P. B. 33,221, May 15, 1946, pages 21 and 22. U. S. Dept. of Commerce, Oflice of Technical Services, Washington, D. C. 

